Low excess air operation of multipleburner residual-fuel-fired furnaces



20, 1963 c. w. SIEGMUND ETAL LOW EXCESS AIR OPERATION OFMULTIPLE-'BURNER RESIDUALFUEL-FIRED FURNACES Original Filed April 12,1965 FLUE GAS ANALYZER FUEL MANIFOLD AIR MAN F INTERSgInPARING FEEDBACKH CONTROL Charles W. Sie mund John E. Gerror Inventors Potent Ar'rorngyUnited States latent O 3,369,749 LOW EXCESS AIR OPERATION OF MULTIPLE-BURNER RESIDUAL-FUEL-FIRED FURNACES Charles W. Siegmund, Morris Plains,and John E. Gerrard,

Matawan, N.J., assignors to Esso Research and Engineering Company, acorporation of Delaware Continuation of application Ser. No. 447,293,Apr. 12, 1965. This application Feb. 17, 1967, Ser. No. 616,991 5Claims. (Cl. 236-) ABSTRACT OF THE DISCLOSURE A process and apparatus isprovided for establishing and maintaining optimum combustion at a lowlevel of excess air in a multiple-burner furnace or boiler. The air andfuel feed to each individual burner of a multipleburner furnace areadjusted so that the flame temperatures of the burners are essentiallyequal. The flue gas from the furnace is then analyzed and the feed tothe air and fuel manifolds of the multiple-burner furnace is adjusted sothat a desired level of excess air is delivered as determined byanalysis of the flue gas.

Cross-references to related applications This is a continuation ofapplication Ser. No. 447,293, filed Apr. 12, 1965.

The present invention relates to a multiple-burner residual-fuel-firedfurnaces and boilers. In general, it concerns a process and apparatusfor establishing and maintaining optimum combustion at a low level ofexcess air in a multiple-burner furnace or boiler. In particular, itprovides a process for achieving this optimum combustion by achievingthe proper adjustment of the individual burners of such multiple-burnersystems.

Multiple-burner residual-fuel-fired furnaces or boilers are commonlyused in firing power plants. Such furnaces or boilers may have a dozenburners, be as tall as an eight story building and consume up to 400barrels of fuel oil per hour. The residual fuel oils employed in suchburners contain ash (i.e. metallic contaminants that are bothnonvolatile and non-combustible) and sulfur. Ash is troublesome innearly all types of combustion equipment. The most common problem isdeposit formation or slagging which reduces heat transfer, etc. Sulfuris found in varying amounts and chemical combinations in all grades ofresidual fuel oil. Some heavy fuels from high sulfur crudes contain asmuch as 5 wt. percent sulfur, 'while others from low sulfur crudes mayhave less than 1 wt. percent. Typical is a No. 6 oil having an averagesulfur level of 2.5 Wt. percent. When sulfur-containing fules areburned, part of the sulfur is converted to sulfur trioxide which in turncauses surfuric acid corrosion.

It is well known that the operation of residual-fuel-fired systems issubstantially improved in terms of reducing superheater slagging andcorrosion of both superheaters and their preheaters by carrying .out thecombustion at levels of low excess air only slightly higher thanstoichiometric, e.g. 0.013% excess. This is an effective method ofoptimizing combustion, but in the past has been limited in applicationbecause smoke formation and poor combustion usually result before thelow excess air combustion is achieved. One of the problems associatedwith achieving this low excess air? combustion is assuring that theindividual burners of a multipleburner system are each operating at thesame level of excess air. For instance, if half the burners in amultipleburner system operate at 6% excess air, and the remainderoperate at stoichiometric conditions, the flue gas analysis of the stackgases for the percent oxygen will show the overall combustion to be at3% excess air (i.e. optimum Patented Feb. 20, 1968 combustionconditions). However, conditions are actually poor in terms of smoke andcorrosion, since the burners operating at or slightly belowstoichiometric conditions will produce dense smoke, and those operatingat 6% excess air will tend to cause superheater and air preheatercorrosion. I

According to the present invention, there is provided a process andapparatus which assure that the individual burners of a multiple-burnersystem are each operating at the same level of excess air and thusafford a practical means of realizing the inherent benefits of lowexcess air combustion.

In the present invention the air and fuel feed to each individual burnerof a multiple-burner furnace are adjusted so that the flame temperaturesof the burners are essentially equal. Subsequently, the overall level ofair or fuel feed to all burners is adjusted so that the furnace operatesat the optimum level of excess air.

The initial step of adjusting each individual burner so that the flametemperatures associated with all the burners are essentially equal canbe accomplished in various ways. This step may be conveniently referredto as balancing the burners. While the burners can be balanced by any ofseveral methods, all of the methods involve a manipulation of the air orfuel feeds to the individual burners. The Various methods used tobalance the burners differ primarily in the different temperaturesensing devices used to determine flame temperatures. That is, differenttemperature sensing devices can be employed to provide the informationabout the flame.temperatures. Among these various devices that can beused are thermocouples, photopyrometers (employed both within the flameand immediately adjacent to it), ultraviolet analyzers, and microwaveanalyzers. In a preferred embodiment of the present invention a closedtube photopyrometer is employed as the temperature sensing device.

A closed tube photopyrometer includes a sensing element which is ahollow tube of a refractory material, such as alumina, closed at one endplus an optical detection device i.e. optical detector, for measuringthe intensity of radiation; for example a photoresistive cell such as, acadmium selenide or cadmium sulfide cell, a photomultiplier, or the likecan be employed as the optical detector. The closed end of the tube isplaced into or adjacent to the flame where it quickly achieves atemperature essentially equal to the temperature of the flame. It thenglows, and emits radiation, which is characteristic of this temperature.When the optical detector is placed adjacent to the open end of thetube, the detector will produce a signal which will change as a functionof the intensity of the radiation. When the optical detector is aphotoresistive cell, its resistance will change as a function of theintensity of the radiation. This resistance can be measured by anystandard means such as a Wheatstone bridge.

The purpose of the closed tube is to shield the detector from anyincident radiation exceptv that from the end of the tube and to smoothout the rapid fluctuations in temperature characteristic of a singlelocation in a flame to give a steady temperature characteristic of theoverall flame. Without the tube, background radiation which is notcharacteristic of the flame temperature would aflFect the detector. Inaddition, the resistance of the detector would fluctuate rapidly as itresponded to the flicker the desired level of excess air combustion ateach burner is effected by adjusting the air and/or fuel flow to theentire assembly i.e., all burners. This may be done by various methods.For example, if the multiple-burner furnace is provided with an airmanifold and a fuel manifold which substantially evenly distribute theair and fuel feeds, respectively, to the individual burners, the airand/or fuel flow to all the burners may be adjusted by manipulation ofthe manifold controls so as to achieve overall air/ fuel ratiooptimization. This overall optimization is determined by flue gasanalysis. Flue gas analysis is a well-known procedure to those skilledin the art and need not be described further.

In one embodiment of the present invention the process and apparatusdescribed herein is employed to automatically control desired levels ofexcess air in multiple-burner furnaces.

In a preferred embodiment of the present invention a temperatureintercomparison technique is employed. This technique is far moreaccurate and much less costly than techniques based upon measurement ofabsolute temperature values. The technique compirses providing a means,hereinafter referred to as an intercomparing unit, for comparing theflame temperatures of the various burners directly with one another asopposed to first measuring the absolute temperature values of theindividual flames and then comparing the measured temperatures.

For example, the intercomparison unit can be a series of calibratedoptical detectors placed adjacent to the ends of sensing elements (e.g.hollow alumina tube closed at one end) which are inserted into theflames of individual burners in a multiple-burner furnace. The air/ fuelratio to each burner can be adjusted so that, for example, theresistances of all of the optical detectors are essentially equal. Whenthis has been accomplished, the burners are balanced, i.e. the air levelat all burners is essentially equal, and the overall level can beadjusted to the optimum level.

An alternate and preferred method is to insert a series of matched fiberoptics into the open ends of the tubes. The radiation issuing from theother end of each in turn is permitted to strike a Single opticaldetector. While the radiation from the tube inserted into the firstflame is striking the optical detector the resistance is noted. Then theradiation from the second is permitted to strike the optical detectorand the air/fuel ratio to the second burner is adjusted to give the sameresistance as the first. This sequence is followed until all burnersgive the same resistance. By this scheme the need for calibrated opticaldetectors is eliminated because the same detector is used for all of theflames.

The invention is further understood by reference to the accompanyingdrawing in which FIGURE 1 is a schematic diagram of a process andapparatus for automatically controlling the level of excess aircombustion in a four-burner furnace according to the present invention.

With reference to FIGURE 1 there is shown a furnace 1 equipped with fourburners 2. Sensing elements 3, e.g. a hollow alumina tube closed at oneend, extend into or near each burner flame 14 where the sensing elements3 glow and produce radiation according to the well-known Plancks law forradiation from a black body. Air and fuel are supplied to each burner 2through the respective control valves 4. Fuel is supplied to eachcontrol valve 4 from a fuel manifold 5. Air is supplied to each controlvalve 4 from an air manifold 6. A master control valve 7 regulates theair/fuel ratio to the fuel manifold and the air manifold 6. A flue gasanal zer 8 is located in the furnace stack 13 and analyzes the mixtureof flue gases from the four burners 2. Signals from the sensing elements3 are conducted through suitable conduit means 15, e.g. fiber optics,and compared at an intercomparing unit 19 and in the event of imbalanceintercomparing unit activates an imbalance signal, e.g., an electricalsignal, to suitable feedback control 11 to effect a balance of theimbalanced burner. The overall excess air level is determined by fluegas analyzer 8. Master feedback control 12, which includes a timer (notshown), periodically compares the overall excess air level as determinedby analyzer 8 with the desired level, and in the event of a differenceactivates an error signal through suitable conduit means 17 to mastercontrol valve 7 which adjusts the air and/or fuel feeds to theirrespective manifolds bringing the overall level of combustion to thedesired level. Each control valve 4 and master control valve 7 are shownas single valves, however two separate valves, one on the fuel supplyline and one on the air supply line, can also be employed at any or allof these points in the system. The timer in master feedback control 12also periodically actuates through a suitable conduit 16, such as anelectrical wire, feedback control 11, allowing feedback control 11 torespond to any imbalance signal from intercomparing unit 10 and therebyeffect a balance of the imbalanced burner. The timer, of course,coordinates the steps of balancing and the step of adjusting the overalllevel of combustion so that these steps do not take place concurrently.Both the balance and the overall level are thus maintained automaticallyand indefinitely assuring achievement of desired low excess air levelsand its inherent benefits.

As mentioned supra, a suitable intercomparing unit provides a means forcomparing the individual flame temperatures of the various burnersdirectly with one another, without measuring absolute temperaturevalues. Such means are apparent to those skilled in the art. In apreferred embodiment, radiation from the various sensing elements isconducted to the intercomparing unit through a series of matched fiberoptics (one fiber optic for each burner) and the unit includes a singleoptical detector, means for permitting the radiation from only one fiberoptic at a time to strike the optical detector, means for noting theresistance of the optical detector, means for comparing resistancelevels of the optical detector and means for producing an imbalancesignal representative of different optical detector resistance levels.The matched fiber optics can be fiber optics such as LG-3, obtained fromAmerican Optical Company, which are matched or calibrated so that theywill conduct equal amounts of radiation.

The optical detector can be a cadmium selenide photoresistive cell suchas Catalogue No. CL-603AL, obtained from Clairex Corporation.

The means for permitting the radiation from only one fiber optic at atime to strike the optical detector can be an optical chopper which isfurther described in a cofiled application, Ser. No. 447,292, filed Apr.12, 1965. The optical chopper consists of an opaque disc with a wedgeremoved of an angle=360ln where :2 equals the number of burners in thefurnace. The disc is driven by a small electrical motor.

The means for noting the resistance of the optical detector can be aWheatstone bridge.

The means for comparing the resistance levels of the optical detectorand the means for producing an imbalance signal are well known in theart. For example, the resistance levels of the optical detector may beviewed as voltage signals and such signals can be compared with oneanother to produce an imbalance signal which is sent to suitablefeedback controls.

Suitable feedback controls are well known in the art. For example, suchcontrols can include an amplifier for amplifying the imbalance signalfrom the intercomparing unit and proportioning control valves (e.g.electrical or pneumatic) on the air and/or fuel feeds to the individualburners.

Suitable master feedback controls are likewise well known in the art.For example, such controls can include a timer for coordinating andperiodically actuating the feedback controls, the flue gas analyzer, andthe master control valve(s) to the air and/or fuel feeds; comparingmeans, such as a differentiator, for comparing the measured with thedesired overall air level in the flue gas and for producing an errorsignal; amplifying means to amplify error signals; and means to transmitamplified error signals to suitable control valves. Usually, masterfeedback controls will include means for allowing an operator to effecta step change in the fuel supply when for example, steam outputrequirements change.

While the embodiment shown in FIGURE 1 describes a preferred process andapparatus for automatically controlling the level of excess aircombustion at each individual burner of a multiple-burner system, theprocess of the present invention can also be employed without theautomatic control features. That is, an operator may take theinformation obtained from a flue gas analysis and manually adjust theair/fuel ratio by adjusting the air and/ or fuel master valve. In asimilar manner, the operator may also use the information from theintercomparison unit to manually operate the air and/or fuel controlvalve at each burner (i.e. to balance the burners).

While a general description has been given and preferred embodiment ofthe present invention has been described and illustrated, it is to beunderstood that various modifications and adaptations thereof can bemade without departing from the spirit of the invention and the scopethereof.

For instance, while a photoresistive cadmium selenide cell has beendescribed as the optical detector employed in the preferred embodiments,a photoemissive device (eg a photomultiplier) can be used as thedetector. Similarly, While in the described preferred embodiment, thesystem is provided with air and fuel manifolds, other means can be usedto supply substantially equal amounts of air to each individual burnerand to supply substantially equal amounts of fuel to each individualburner.

What is claimed is:

1. A process for the control of low excess air combustion in amultiple-burner residual-fuel-fired furnace provided with an airmanifold and a fuel manifold, which comprises:

(a) comparing the flame temperature of each individual burner;

(b) adjusting the ratio of air and fuel delivered to each individualburner so that the flame temperature of said burners are essentiallyequal;

(c) analyzing the flue gas from the furnace; and

(d) adjusting the feed to the air manifold and the fuel manifold so thata desired level of excess air is delivered as determined by flue gasanalysis.

2. A process according to claim 1 wherein said desired level of excessair does not exceed about 3%.

3. Apparatus for the automatic control of combustion in amultiple-burner residual-fuel-fired system comprising (a) a temperaturesensing means adapted to measure the individual flame temperature ofeach of said burners; (b) an intercomparing unit adapted to compare theindividual flame temperatures directly with one another and furtheradapted to produce imbalance signals representative of any differencebetween said temperatures, (c) feedback controls responsive to saidimbalance signals and adapted to control the air-fuel feed to eachindividual burner; (d) a flue gas analyzer adapted to produce a firstsignal representative of the overall air level in the stack of saidsystem; and (e) master feedback controls responsive to said first signaland adapted to compare said first signal with a second signalrepresentative of the desired overall air level and further adapted toproduce an error signal and to control the air-fuel feed to all burnerssimultaneously.

4. An apparatus according to claim 3 wherein said master feedbackcontrols are further adapted to actuate periodically said feedbackcontrols.

5. An apparatus according to claim 3 wherein said master feedbackcontrols control the. air-fuel feed to all burners simultaneouslythrough an air manifold and a fuel manifold.

No references cited.

EDWARD I MICHAEL, Primary Examiner.

